Liquid crystal display devices, organic electroluminescence (EL) display devices and the like have an image-forming mechanism including polarizing elements as essential components. For example, therefore, in a liquid crystal display device, polarizing elements are essentially arranged on both sides of a liquid crystal cell, and generally, polarizing plates are attached as the polarizing elements. Besides polarizing plates, various optical elements for improving display quality have become to be used in display panels such as liquid crystal panels and organic EL panels. Front face plates are also used to protect image display devices such as liquid crystal display devices, organic EL display devices, CRTs, PDPs or the like to provide a high-grade appearance or a differentiated design. Examples of parts used in image display devices such as liquid crystal display devices and organic EL display devices or parts used together with image display devices, such as front face plates, include retardation plates for preventing discoloration, viewing angle-widening films for improving the viewing angle of liquid crystal displays, brightness enhancement films for increasing the contrast of displays, and surface treatment films such as hard-coat films for use in imparting scratch resistance to surfaces, antiglare treatment films for preventing glare on image display devices, and anti-reflection films such as anti-reflective films and low-reflective films. These films are generically called optical films.
When such optical films are bonded to a display panel such as a liquid crystal cell, an organic EL panel and the like, or bonded to a front face plate, a pressure-sensitive adhesive is generally used. In the process of bonding an optical film to a display panel such as a liquid crystal cell, an organic EL panel and the like, or to a front faceplate or bonding optical films together, a pressure-sensitive adhesive is generally used to bond the materials together so that optical loss can be reduced. In such a case, an adhesive optical film and a pressure-sensitive adhesive layer previously formed on one side of the optical film is generally used, because it has some advantages such as no need for a drying process to fix the optical film and the like.
Optical films are vulnerable to shrinkage or expansion under heating or humidifying conditions. If the adhesion between an optical film and a pressure-sensitive adhesive is low, the optical film can lift or peel from the pressure-sensitive adhesive layer. Particularly in in-vehicle applications such as car navigation systems, liquid crystal panels are required to have higher durability, and in such applications, optical films are exposed to high shrinkage stress and can more easily lift or peel. Specifically, for example, even if there is no problem in a reliability test performed at about 80° C. for TVs or the like, a problem such as lifting or peeling can easily occur in a reliability test performed at about 95° C. for in-vehicle products such as car navigation systems. After an adhesive optical film is bonded to a liquid crystal display, if necessary, the optical film is temporarily peeled off and then bonded again (subjected to reworking). In this process, if the adhesion between the optical film and the pressure-sensitive adhesive is low, the pressure-sensitive adhesive can remain on the surface of the liquid crystal display, so that a problem can occur in which reworking cannot be performed efficiently or in which if the edge of the adhesive optical film comes into contact with a worker or something adjacent to it in the process of cutting, feeding, or handling it, the pressure-sensitive adhesive can be chipped off of the edge portion, which can easily cause a display failure in the liquid crystal panel. To solve these problems, a technique for increasing adhesion between an optical film and a pressure-sensitive adhesive layer is performed, which includes applying an anchor layer to the optical film and then applying the pressure-sensitive adhesive thereto.
On the other hand, the pressure-sensitive adhesive layer is required not to cause the adhesive to form a defect in an endurance test, which is usually performed as an accelerated environmental test under heating and humidifying conditions or other conditions. Unfortunately, when an anchor layer is disposed between an optical film and a pressure-sensitive adhesive layer, there is a problem in that solvent cracking occurs on the anchor layer-coated surface side of the optical film during an endurance test. Particularly in a reliability test performed at about 95° C. for in-vehicle products such as car navigation systems, solvent cracking significantly occurs in some cases, even if no solvent cracking occurs in a reliability test performed at about 80° C. for TVs or the like.
Patent Document 1 listed below discloses an adhesive optical film, a pressure-sensitive adhesive layer, and an anchor layer interposed between the optical film and the pressure-sensitive adhesive layer, wherein the anchor layer is obtained by applying an anchor layer-forming coating liquid containing a polyamine compound and a mixed solvent of water and an alcohol, and by drying the coating liquid. Concerning such an adhesive optical film, however, the composition of the anchor layer-forming coating liquid and the drying conditions are not specifically studied for the purpose of solving the problem of solvent cracking that occurs on the anchor layer-coated surface side of the optical film during an endurance test.
Patent Document 2 listed below discloses an adhesive optical film, a pressure-sensitive adhesive layer, and an anchor layer disposed between the optical film and the pressure-sensitive adhesive layer, wherein the anchor layer is obtained by applying an anchor layer-forming coating liquid containing an oxazoline group-containing polymer and a mixed solvent of water and an alcohol, and by drying the coating liquid. Patent Document 2 also discloses a specific example in which the anchor layer-forming coating liquid is dried under the conditions of a drying temperature of 40° C. and a drying time of 120 seconds. Patent Document 3 listed below discloses an adhesive optical film, a pressure-sensitive adhesive layer, and an anchor layer disposed between the optical film and the pressure-sensitive adhesive layer, wherein the anchor layer is obtained by applying an anchor layer-forming coating liquid composed of an aqueous solution containing a polyurethane resin and a water-soluble polythiophene-based conductive polymer, and by drying the coating liquid. Patent Document 3 also discloses a specific example in which the anchor layer-forming coating liquid is dried under the conditions of a drying temperature of 80° C. and a drying time of 120 seconds. However, it has been found that these drying conditions are not enough to prevent the solvent cracking described above and there is room for improvement.
Patent Document 4 listed below discloses an adhesive optical film, a pressure-sensitive adhesive layer, and an anchor layer disposed between the optical film and the pressure-sensitive adhesive layer, wherein the anchor layer is obtained by applying an anchor layer-forming coating liquid containing ammonia and an aqueous dispersion-type polymer and by drying the coating liquid. Patent Document 4 also discloses a specific example in which the anchor layer-forming coating liquid is dried under the conditions of a drying temperature of 50° C. and a drying time of 60 seconds. However, if the content of ammonia in the anchor layer is high, for example, when a polarizing plate is used as the optical film, the polarizing properties of the polarizing plate can change in a high-temperature or high-humidity environment. This affects the optical properties and sometimes makes it impossible to achieve high durability in a high-temperature or high-humidity environment.
As described above, the conventional techniques provide no example in which attention is focused on the problem of solvent cracking that occurs on the anchor layer-coated surface side of the optical film, and to solve the problem, it is necessary to make a further study.